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Diagn Microbiol Infect Dis. 2012 Dec;74(4):412-4. doi: 10.1016/j.diagmicrobio.2012.08.025. Epub 2012 Oct 25.

Use of a surfactant (polysorbate 80) to improve MIC susceptibility testing results for polymyxin B and colistin.

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JMI Laboratories, North Liberty, IA, USA.


Accurate determination of in vitro activity for polymyxin class agents has consistently been a problem due to their physical-chemical characteristics that can be influenced by the constituents of reference and/or standardized susceptibility testing methods. We evaluated the impact of using polysorbate 80 (P-80), a surfactant, in reference broth microdilution (BMD) methods when testing polymyxin B and colistin against 247 clinical strains of Enterobacteriaceae (124 strains), Acinetobacter spp. (60 strains), and Pseudomonas aeruginosa (63 strains). All testing was performed in frozen-form BMD panels with and without 0.002% P-80. MIC results for both polymyxins were generally 4- to 8-fold lower when P-80 was added to the testing broth compared to Mueller-Hinton broth without the surfactant. Decreases were greatest in organisms having MIC values at ≤2 μg/mL and among Acinetobacter spp. Polymyxins should be tested with P-80 to more accurately assess the potencies of these agents necessary to treat multidrug-resistant Gram-negative bacilli.

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